This invention relates to rotary digital encoding apparatus especially useful as a channel selection control in multi-channel radio communications equipment, particularly two-way radio transceivers for citizens' band, public radio, industrial and business applications. However, in its broader aspects the present invention is applicable to any switching requirement in which a desired operating frequency or other function is selectable by means of a digital code.
As the popularity and need for two-way radio service grows, particularly in the public sector known as the "Citizens Radio Service", overcrowding of available channels restricts the usefulness and availability of the service for all private individuals who desire it. Consequently, to expand the availability of the Citizens' Radio Service in the public interest, the Federal Communications Commission is contemplating expanding the number of channels available from the original twenty-three to some greater number which may be as high as thirty-six, forty, or seventy channels. Modern, economical design of two-way radio transceivers is seeing the rapid adoption of frequency synthesizer techniques employing digital phase-lock loop circuits for generation of the large number of discrete frequencies required in such multi-channel transceivers using a single voltage controlled variable frequency crystal stabilized local oscillator in the superhetrodyne receiver incorporated therein. Thus, the associated circuiting developed a voltage corresponding to a binary or coded binary decimal signal selected by a digital encoder switch, the voltage adjusting the local osciellator to the frequency needed to receive the desired radio signal. The availability of high performance and low cost integrated circuits for forming said phase-lock loop circuits plus the limited availability of quartz crystals have accelerated the effort to produce multi-channel synthesizers using only one or two crystals instead of the considerably larger number (twelve to twenty) formerly employed in conventional "crystal synthesizer" designs.
The digital encoder switch used to select a desired channel-indicating coded signal to be fed to the phase-lock loop circuit has been a mechanical multi-pole wafer switch. In such case, if the synthesizer is required to produce thirty different frequencies, at least a five pole, thirty position binary coded wafer switch would be required. As the number of channels increase, additional poles are required on the switch so that the binary number represented by the number of poles (digits) is equal to or greater than at least the number of channels desired. A frequency synthesizer as described is capable of generating discrete frequencies which are separated from each other by a fixed amount. The spacing between frequencies is determined by the specified channel separation of the particular communications service for which it is designed. The Citizens' Radio Service, for instance, employs ten kilohertz channel spacing for AM double sideband operation and five kilohertz for single sideband supressed carrier operation. If the channels are consecutively assigned on five or ten kilohertz steps, then the number of poles in the binary switch equals the number of binary digits required for the desired number of channels to be covered. If the channels are not assigned on a consecutive frequency interval basis, but certain channels are skipped for use by other services or otherwise prohibited, then it is the total number of intervals included between the lowest to the highest frequency channels desired that determines the number of poles on the switch. In the Citizens' Radio Service, for instance, a proposed expansion to 64 channels covering the frequency range 26.965 MHz to 27.515 MHz includes 110 five kilohertz channels. Thus, although only 64 channels might be authorized in this range, an eight pole binary switch would be required rather than a six pole switch for 64 channels covered on consecutive five kilohertz channel spacings.
Conventional rotary wafers designed for digitally coding a frequency synthesizer as described above become more complicated and costly as the number of channels increases. Futhermore, the limitation on the number of channels that can be built into such a switch if it is to be of reasonable size appears to be about forty. A fairly long lead time is required to tool up for such conventional rotary switches since each rotor design is unique to the assigned channel spacing, especially when channels are not assigned on a consecutive basis.
Conventional rotary wafer switches also have the disadvantage of poor reliability as their complexity increases. Furthermore, such switches require connection by means of wires, solder tabs or multi-pin plugs and sockets to the circuits they control. Thus, there is a significant labor cost associated with their use. This becomes a particular disadvantage if it is desired that the switch be changeable for field modification of transceivers to prevent their obsolescence when new channel frequencies are permitted and assigned. Thus, one of the main advantages of phase-lock loop frequency synthesizer designs in two-way multi-channel transceivers would be largely offset, namely, the advantage of flexibility of providing customer modification service at low cost to include new channels of an expanded service to present expensive equipment obsolescence.
Therefore, it is an object of this invention to provide a digital encoding switch means of simple design, lower cost and greater reliability than previous designs of the rotary wafer switch type.
Another object of this invention is to provide a rotary digital encoding switch of small size that could accomodate a larger number of channel selections for a phase-lock loop frequency synthesizer circuit than is possible within the practicability of the design of conventional rotary wafer switches of similar size.
A further object of this invention is to provide a rotary digital encoding switch that could accomodate a larger number of channel selections by merely substituting for the rotatable part thereof another inexpensive rotatable part which is automatically indexable to a correspondingly greater number of channel selection positions.
Another object of this invention is to provide a rotary digital encoding switch as described in which the encoding portion can be manufactured at very low cost, such as by injection molding, and in which the programming means thereof could be designed and manufactured with very short lead time, such as if produced from artwork by protographic means.
The present invention is an improvement over and a new use for the type of encoder switch disclosed in U.S. Pat. No. 3,622,793. This type of encoder suited includes a cylindrical encoder drum made of a light-opaque material provided with circumferentially spaced groups of light-transparent areas with therein for forming different groups of light patterns constituting the digits of a straight binary or binary decimal code. The drum is movable to various discrete positions where the various groups of light-transparent areas are brought opposite a light pattern reading station where an array of photocells (or other light pattern reading means) is provided. Light is directed through the light-transparent areas of the drum at the light pattern reading station, and the light pattern reading means thereat provides a pattern of electrical signals indicating the desired information represented by the position of the encoder drum. (A rotary digital encoding unit using a flat annular coding disc forming part of a tuning knob assembly of different construction from the present invention is presently being made by Tektronix, Inc.)